Laser diodes based on metallic nitrides, such as GaN and other Group III-V semi-conductors are of great commercial interest. These diodes have a wide range of uses and potential uses for visible light and UV applications as well as for high density data storage systems. Typically, such laser diodes operate in the blue region of the optical spectrum.
These laser diodes are typically fabricated by depositing, on a sapphire substrate, a number of layers of GaN-based semi-conductors. C-plane and A-plane are the primary orientations employed for the sapphire substrates in the manufacture of such diodes.
In order to generate an inductive emission in a semi-conductor device such as a laser diode, a pair of optical facets are required to enclose and reflect the light on the two sides of the light emitting area and to form a resonator between the facets.
These facets (or mirrors) are formed at the end of the laser cavity. The facets are typically created by dry etching or, as is preferred, by cleaving the sapphire substrate and GaN layers. In order to form cleaved facets in laser diodes, smooth and vertical cleave planes are required in the region where the optical modes are contained. This region is usually limited to the epitaxial layers. To form facets that are both smooth and vertical across the epitaxial layers, the crystal structure of the substrate and the epitaxial layers must have vertical planes.
Ideally, the cleavage occurs along the crystal planes of the GaN layers leaving smooth facets that are perpendicular to the direction of light propagation in the laser.
Generally, the cleaving is accomplished by cutting the back side of the sapphire substrate (that is, the side which does not have the GaN layers formed thereon). This cutting is generally effected by a dicing saw or a scribe to form elongated grooves in the sapphire substrate. The troughs of these grooves then become cleavage points.
It is not easy to cleave sapphire. In order to achieve cleaving, it is generally necessary to thin the sapphire substrate down to about 100 μm (from about 350–400 μm). Also, cleavage will be easier and result in a better product if the cleavage plane of the sapphire substrate matches the cleavage (crystal) planes of the GaN layers. However, before cutting the grooves (eg. with a dicing saw or scribe) it is often difficult to match (or align) the cleavage plane of the sapphire substrate with that of the GaN layers. The crystal planes of the GaN layers are generally not well aligned with the crystal planes of the sapphire. As noted above, upon cleaving, the facet of the substrate may not be properly aligned with the facet of the GaN layers. In order to overcome these problems, additional, time-consuming steps often need to be taken which may include polishing of the facets.
In order to improve the quality of mirrors formed from such cleaved facets, the application of high reflectivity coatings has been used. Also, dry etching methods have been utilised, such as reactive ion etching (RIE) and reactive ion beam etching (RIBE). These methods have been used to smooth the facets without having to resort to mechanical polishing. However, these etching processes have proven to be time-consuming and difficult to perform.
Due to greater ease of manufacture and the formation of anisotropic facets, cleaved laser mirrors are preferred to the dry etched facets. Additionally, cleaved facets do not require any dry etching for mirror formation.
Laser diodes relying on cleaved or etched GaN/air facets are quite sensitive to external optical feedback. One significant problem of forming light reflectivity facets is the low refractive index of GaN (at 400 nm). This leads to a significantly reduced mirror reflectance which can be further decreased by any interface roughness (which is typical of dry chemically etched laser facets and cleaved facets). It has been found that facet surface roughness of about 20 nm can reduce the reflection of the laser mode by an order of magnitude.
Conventional laser devices, made on GaAs/InP substrates are largely fabricated by cleaving methods, owing to the ease of cleaving along the well defined crystal planes ([110] and [1-10]). This results in highly anisotropic and smooth facets for the laser resonator cavity. However, it can be very difficult to neatly cleave GaN-based epilayers on sapphire substrates because the cleavage plane of the sapphire generally does not coincide with the cleavage plane of the GaN layers.
Accordingly, this invention is directed towards an improved method of cleaving GaN layers on a substrate such as sapphire.